In code division multiple access (CDMA) communication systems, a plurality of base stations cover each a certain geographic area and provide communication services to users within such areas. Characteristic for CDMA-systems is that the common transmission medium is shared between different users by assigning specific and unique code sequences to the uplink and downlink channels between base station and user equipment. These code sequences are used by the transmitters to transform signals into wideband spread spectrum signals. In the receiver units, base station or user equipment, said wideband signals from a specific transmitter are retransformed into the original bandwidth by using the same code sequence as that transmitter while signals marked with different codes remain wideband signals and, thus, are interpreted by the receiver as part of the background noise.
An aspect specific to the multiple access technique used in CDMA relates to the fact that all users transmit wideband signals potentially at the same time and using the same bandwidth. Thus, one limiting factor in CDMA-based systems is a decreasing signal-to-noise ratio in the communication channels due to an increasing interference level caused by other transmitters. Said signal interference level per cell results from transmitters within said cell as well as from transmitters in the neighbouring cells. From this it becomes apparent that power control, both of uplink signals and downlink signals, is crucial in order to achieve a maximised system capacity and high quality of services. This implies for the uplink signals that the transmission power of the user equipment must be controlled, e.g., in order to avoid that users close to their serving base station dominate over users at a longer distance and in order to allow an optimised number of users within the limitations of the acceptable interference level. For the downlink signals power control is required, i.a, in order to minimise the interference to other cells and, accordingly, to compensate against interference from other cells. By this means, a base station is able to serve users preferably at any location within the cell and without a too strong contribution to the interference level of the neighbouring cells. Yet another important aspect for downlink power control relates to the fact that increasing power values lead to a spreading of the signal spectrum. Therefore, in order to avoid a spreading of the total downlink signal spectrum outside the allowed frequency range that is reserved for the downlink signals of a CDMA-based communication system, the total allocated downlink power for a base station must be limited to a maximum power level. This can be done, e.g., by means of a digital power clipping. However, it is a drawback of this solution that all users are affected equally.
As thus the downlink transmission power of a base station is a limited resource there is a need for methods to allocate appropriate power levels to the various downlink channels in such a way that the total allocated power remains below said maximum power level. One solution for said downlink power control relates to a fixed allocation of a maximum downlink power per user such that the sum of the allocated power levels remains below the maximum threshold. This minimises the risk that said maximum power level is exceeded but results in poor system capacity due to a lack of flexibility.